1. Field of the Invention
The present invention relates to a method for controlling laser power of an optical disk drive, and more particularly, to an automatic control method using a feedback mechanism to steady laser power of an optical disk drive.
2. Description of the Prior Art
Management of documents is an important concern for both companies and individuals. In the past, all documents were printed or written on paper. This method is very inconvenient as the number of documents continues to increase. As computer technology develops day-by-day, digital data is widely used and stored in computer storage media. In order to make storing digital data more convenient, many data storage devices have been developed such as writable compact disk drives. The optical disk drive has a low cost and a light volume, but a high storage capacity, making storing data more convenient.
Please refer to FIG. 1. FIG. 1 is a perspective view of a laser power control device 10 according to a prior art optical disk drive. The laser power control device 10 comprises a microprocessor 12, a digital to analog converter (DAC) 14, an analog front end (AFE) circuit 15, and an optical pick-up unit 18. The microprocessor 12 outputs a control signal 20 to the DAC 14. The DAC 14 converts the digital control signal 20 into an analog control signal and then transmits the converted analog control signal to the analog front end circuit 15. The initial control signal 20 is a reference signal that is used to control the optical pick-up unit 18 to generate a suitable laser power. The analog front end circuit 15 generates a corresponding writing voltage 22 according to the control signal 20 so as to control the laser power outputted from the optical pick-up unit 18. That means the optical pick-up unit 18 can output different laser power levels according to different writing voltages 22. When the optical pick-up unit 18 outputs laser beam, the laser beam is bisected by a prism. A greater part of the laser beam goes through the prism along the original optic path and then outputs to an optical disk. The smaller part of the laser beam is refracted by the prism and then arrives to a front photo diode. The front photo diode converts the laser beam into a watching signal 24. The watching signal 24 is used to monitor a status of the laser power outputted from the optical pick-up unit 18. If the optical pick-up unit 18 outputs a greater laser power, the power of the laser beam that is refracted by the prism will also be greater. Therefore, users can know the variation of the laser power outputted from the optical pick-up unit 18 according to the variation of the watching signal 24. The analog front end circuit 15 provides a reference voltage 26 to be a reference voltage of the watching signal 24. The analog front end circuit 15 comprises a automatic power control (APC) circuit 16. The automatic power control circuit 16 adjusts the writing voltage 22 according to a difference amount between the watching signal 24 and the reference voltage 26 so as to steady the laser power outputted from the optical pick-up unit 18. For example, the decided value of the laser power outputted to the optical disk from the optical pick-up unit 18 is X. When laser power of X is outputted, a watching signal 24 of VI voltage is generated at the same time. If the reference voltage 26 provided by the analog front end circuit 15 is V2, then the difference amount between the watching signal 24 and the reference voltage 26 is V1−V2. When the analog front end circuit 15 receives a watching signal 24 which has a voltage greater than V2 for the instable output of the optical pick-up unit 18, that means the laser power outputted from the optical pick-up unit 18 is greater than the decided value X at that time. Therefore, the analog front end circuit 15 use the automatic power control circuit 16 to decrease the writing voltage 22 so as to decrease the laser power outputted from the optical pick-up unit 18 at the same time. Therefore, the laser power control device 10 of the prior art optical disk drive uses the difference amount between the watching signal 24 and the reference voltage 26 to finely tune the writing voltage 22 so as to make the optical pick-up unit 18 output stable laser power.
However, the laser power control device 10 only forms a feedback mechanism between the analog front end circuit 15 and the optical pick-up unit 18 to steady the laser power outputted from the optical pick-up unit 18. When the optical pick-up unit 18 outputs the unsuitable writing power, it is unable to adjust the writing voltage 22 generated by the analog front end circuit 15 through the feedback mechanism. For example, when error caused by the laser power control device 10 makes interference, the optical pick-up unit 18 will output undesirable writing power. However, the laser power control device 10 can only use the feedback mechanism formed between the analog front end circuit 15 and the optical pick-up unit 18 to stably output the undesirable writing power. Therefore, the laser power control device 10 is unable to use the undesirable writing power to adjust the writing voltage 22 and to control the optical pick-up unit 18 to output suitable writing power.
In general, the optical disk drive must use an optimum power control (OPC) before writing data onto a disk so as to find out the most suitable writing power for the disk. The orange book has detailed description about the practice of the optimum power control, which is briefly described below.
Please refer to FIG. 2. FIG. 2 is a perspective view of using the optimum power control according to the prior art. When using the optimum power control, the optical pick-up unit 18 of the optical disk drive will first read an indicative optimum writing power P from a lead-in area 29 on the optical disk 28. The indicative optimum writing power P is a writing power suggested by the manufacturer of the optical disk. The optical pick-up unit 18 tests the writing operation in fifteen continuous test blocks 31 inside the test area 30 on the optical disk 28 by using seven writing powers P1˜P7 that are smaller than the indicative optimum writing power P, the indicative optimum writing power P, and the seven writing powers P8˜P14 that are greater than the indicative optimum writing power. In total, fifteen different writing powers are used.
Continuously, the optical pick-up unit 18 reads back the result of writing data into the test blocks 31. The read back signal is an alternating current coupled high frequency signal. The orange book has defined a symmetrical parameter β with regard to the read back signal. If the symmetrical parameter β of the read back signal conforms with the decided demands, the corresponding writing power can be the optimum writing power of the optical disk 28. Fifteen symmetrical parameter values (β) of the fifteen read back signals corresponding to the fifteen writing powers are generated after the optical pick-up unit 18 has read back the writing result of the fifteen writing powers. Then, the optical disk drive compares the fifteen symmetrical parameter values of the read back signals with a target β stored in the lead-in area 29 of the optical disk. The writing power corresponding to a symmetrical parameter value of a read back signal, which has a minimum difference amount from the target β in the fifteen symmetrical parameter values of the read back signals, is the optimum writing power. If all the difference amounts between the fifteen read back signal parameters and the target β are greater than a decided value, none of the fifteen writing powers is suitable to be the practical writing power. In this case, the optical disk drive will use a preset value to be the writing power and write data into the writing area 32 of the optical disk 28.
However, errors are generated between the output laser power of the optical pick-up unit 18 and the writing voltage 22 and the deviation of the laser power is caused due to errors in optical elements and fabricating process. Besides, the resistance effect and noise interference generated by the electronic elements on the printed circuit board of the optical disk drive will influence the actual output voltage. Therefore, an error exists between the output voltage and the original decided value designed by the circuit. The errors generated by the optical pick-up unit 18 and the electronic elements make the optical pick-up unit 18 output a laser power that is too large or too small and exceeds the optimum power control range of the optical disk. The inefficacy of the optimum power control will not only influence the burning efficiency, but also calculate an unsuitable writing power. For example, when using the optimum power control, the optical pick-up unit 18 of the optical disk drive will first read an indicative optimum writing power P from a lead-in area 29 on the optical disk 28. The optical pick-up unit 18 tests the writing operation in fifteen continuous test blocks 31 inside the test area 30 on the optical disk 28 by using seven writing powers P1˜P7 which are smaller than the indicative optimum writing power P, the indicative optimum writing power P, and seven writing powers P8˜P14 which are greater than the indicative optimum writing power. All together, fifteen writing powers are used. However, the error generated by the optical disk drive makes the optical pick-up unit 18 emit a laser beam which deviates from the indicative optimum writing power P. The optical pick-up unit 18 is unable to get a suitable writing power from the fifteen test writing powers. Therefore, the optical pick-up unit 18 must use the heterodyne method or repeat to do the optimum power control to get the writing power. However, it is difficult to get a suitable writing power for the error of the output laser power. If the optical pick-up unit 18 outputs the unsuitable writing power, it will damage the chemical dyes on the optical disk and increase the error rate when reading data from the optical disk, or make the optical disk unusable for storing any further data. In order to calibrate the error, the prior art optical disk drive uses detecting instruments to calibrate the output laser power of the optical disk drive by manpower, through the procedure of testing and recombining the optical pick-up unit 18 many times, so as to make the laser power outputted from the optical disk drive lie inside the optimum power control range. Therefore, not only the production yield cannot be improved, but also the total production cost is increased.